That's only a limit in the number of listings, I think the OP is commenting that the number of sales over month is stagnant.
We've an account with always 1500 to 1700 listings every month, if I don't spend a lot of time listing, we get about the same sales every month. When I put a rocket up my backside, or decide to list the very interesting (for interesting read more expensive) the sales for that month spike. But go back down to normal selling quite quickly.
I think it's just the nature of selling. I had sales targets for years, always selling high value and high quantity. Some brilliant 'blips' but in general the monthly sales were pretty much the same.
Even my seasonal stuff tends to sell at the same rate over the same months every year.
Some things just work out this way, there is something called Pareto's Principle that, in selling equates to the fact that a business receives 80% if it's sales, or it's income, from 20% of it's customers. And it happens, within only slight variables. (eBay likely breaks this rule for small sellers, but you can bet 80% of eBay's sales come from about 20% of it's sellers).
